1. Field of Invention
The present invention generally relates to a parallel coupled resonator filter with open-and-short end and bent resonator to improve the stop-band attenuation rate performance.
2. Description of Related Art
Because of the progress of communication techniques, wireless communication devices are becoming widely accepted now. In various wireless communication system, the band used for transmitting frequency is from the traditional radio frequency (RF) further upgraded to the microwave band. However, regardless of which band is used, eliminating spurious signal is the key point of the performance of wireless communication systems. Therefore, the characteristics of filter become one of major factors of the whole wireless communication system.
Filters used in microwave bands can be made by the different kind of transmission lines, such as microstrip line and CPWline, etc. Generally, resonators are realized by half wavelength of transmission lines with open or short end. The microwave signal is coupled between the resonators. The filter is designed by adjusting coupling coefficients between resonators.
Referring to FIG. 1, it schematically shows a diagram of a conventional three-resonators microstrip line parallel-coupled-resonator filter used in the microwave band. As shown in the diagram, the filter comprises an input port 110, a first resonator 120, a second resonator 130, a third resonator 140, and an output port 150. First, the input port 110 receives an input signal. Then, the signal passes from resonator 120 to resonator 140. Finally, the signal is transferred to output port 150.
The disadvantages of the microstrip coupled line filter are as follows: 1. The frequency response of the conventional microstrip line parallel-coupled-resonator filter is shown in FIG. 2, the stop band attenuation rate is not fast enough to eliminate image signal. 2. In order to obtain a faster stop band attenuation rate, the number of the resonators used has to be increased. However, when the number of the resonators increases, the size of the whole filter will occupy too much space to meet the compact size requirement of system.